114 CHEMICAL ENGINEERING
bucket C is driven by means of wooden rods of convenient length (18 ft.), coupled together. The top of the well.pipe is provided with a stuffing box through which a brass plunger works. If the pump discharges at the surface this plunger is made one-half the area of the bucket, resulting in a uniform discharge. If the pump discharges into an elevated tank the top plunger is made larger so as to throw the effect of the additional head upon the plunger and thereby assist in the up stroke. The well with drop pipe (Fig. 5) differs from the plain tube well in that the brass working barrel is attached to a pipe and is lowered into the well pipe. This is a better arrangement in case of a small supply, as it allows a head of water to collect above the bucket during the down, stroke. It is used in wells that are either wholly or partly drilled into rock and in wells where the casing does not extend low enough to reach the water supply; also in old wells with defective or bent casing.
A centrifugal pump consists of a casing in which revolves an impeller, that is a wheel carrying a number of suitably shaped vanes. The centrifugal force sets up a static head in the casing and a certain velocity is imparted to the liquid. With the discharge valve closed only the static head is noticeable and can be
measured. This is called the shut-off head and is Ha = -£- nearly, where ua = the
peripheral velocity of the impeller in feet per second and g = 32.2 ft. per second. When the discharge valve is opened other forces come into action and it is possible to obtain a head higher than that due to the centrifugal force alone. This higher head is chiefly obtained by the conversion of the velocity into static head.
FIG. 7.—Volute pump. FIG. 8.—Turbine pump.
This conversion can be accomplished in two ways, resulting in the two types: (a) Volute Pump.—The impeller is surrounded by a spiral casing (the volute) of gradually increasing cross-section, Fig. 7.
(6) Turbine Pump.—The impeller is surrounded by a guide ring, provided with a number of suitably shaped channels which receive the liquid in the direction in which it leaves the impeller, guide it to the outside and at the same time reduce its velocity to such an extent that it will readily turn in any direction and flow towards the point of the casing where the discharge may be located, Fig. 8. The volute